According to the Battery Council International (BCI), the state-of-charge of a battery is defined as the amount of electrical energy stored in a battery at a given time expressed as a percentage of the energy when the battery is fully charged. The need for measuring the state-of-charge of a storage battery has recently come to the fore as a result of the increased popularity of electric vehicles (EV's). The problem of measuring the state-of-charge of a storage battery, however, is not new.
What is needed is a metering system analogous to the current fuel gauge systems employed in internal combustion powered vehicles to measure the state-of-charge of the battery storage systems employed with EV's. Such a metering system would provide the operator of an EV with an indication of the amount of charge remaining in the battery and, consequently, an indication of the time remaining before recharging of the battery is necessary.
Conventional internal combustion fuel storage systems are typically monitored by a relatively simple fuel float system. The determination of the amount of fuel remaining in the fuel storage system is easily calculated using simple mechanical and electrical devices. Fuel monitoring systems for internal combustion engines have even advanced to the point that many automobiles now include computers that calculate not only the amount of fuel remaining in the fuel tank, but the miles remaining before refueling is needed.
A simple "Full/Empty" device similar to the fuel monitoring systems employed with internal combustion vehicles, however, is not currently available for battery storage systems, yet such a device will, in the future of the EV, become very necessary. The need for such a device will become particularly pronounced for early model EV's since the availability and location of battery charging facilities will be limited. Without an accurate measure of the remaining charge in a battery storage system, and the time and/or operating miles remaining in the vehicle's storage battery, the operator of an EV could easily become stranded miles from home or available recharging facilities.
For battery storage systems employed in EV's, the operable range of charge required to power such vehicles exists in a narrow range below the fully charged capacity of the battery. According to the BCI, the difference between a fully charged battery and an empty battery can be expressed either in terms of the battery's chemical or electrical characteristics. The data shown in Table 1 is published in the BCI Storage Battery Technical Service Manual and indicates the charge level of a battery in terms of the specific gravity and the open circuit voltage of the battery.
TABLE 1 ______________________________________ Charge Level Specific Gravity Voltage (open circuit) ______________________________________ 100% (full) 1.265 12.68 75% 1.225 12.45 50% (half) 1.190 12.24 25% 1.155 12.06 0% (empty) 1.120 11.89 ______________________________________
Although it is possible to measure the specific gravity of a storage battery using an hydrometer, this is not a practical method for continuously monitoring the state-of-charge for the EV. An alternative method would be to measure the open circuit voltage of the battery using a voltmeter. Unfortunately, a simple measurement of the terminal voltage of a battery that is operating (i.e., being charged or discharged) will not accurately reflect the state-of-charge of the battery. What is required is a metering system that can continuously measure the state-of-charge of the vehicle battery while the vehicle is operating and display the instantaneous state-of-charge to the vehicle operator.